1. Field of the Invention
The present invention relates to an optical pickup device for reproducing information stored in an optical disc such as a DVD (Digital Versatile Disc) or a BD (Blu-ray Disc) and an optical disc apparatus using the optical pickup device.
2. Description of the Related Art
An optical disc such as a DVD or a BD is configured as a multi-layer disc including two or more recording layers to increase its recording capacity. It is expected to more and more increase the recording capacity by increasing the number of recording layers in the future. However, it is unavoidable to reduce the reflectance per layer in order to increase the number of recording layers and to obtain signal light from each recording layer. As a result, such a problem may occur that a sufficient S/N ratio of a reproduction signal is not ensured and hence stable reproduction is not attained. In addition, since high-speed reproduction becomes desirable with an increase in capacity, it becomes inevitable to increase the S/N ratio of the reproduction signal.
As a technique involving an increase in S/N ratio of a reproduction signal, a technique of splitting light emitted from a light source into several streaks of light before an optical disc is irradiated with it, synthesizing light which is not used for irradiation of the optical disc with light reflected from the optical disc to cause interference between them so as to amplify the amplitude of a detection signal from the optical disc is known. For example, Japanese Patent Application Laid-Open No. 2008-65961 discloses a configuration that an optical beam emitted from a light source is divided into first and second optical beams, the first optical beam is condensed on an optical disc to irradiate the disc with it, signal light reflected from the optical disc is guided to four detectors, and the second optical beam is not condensed on the optical disc and is guided to the four detectors as reference light, then, the signal light is made to optically interfere with the reference light in an out-of-phase state on the four detectors and all or some of outputs from the four detectors are arithmetically operated selectively to obtain a reproduction signal.